1. Field of the Invention
This invention relates to semiconductor chip packages, and more specifically to low-pin-count chip packages and manufacturing methods thereof.
2. Description of the Related Art
FIG. 1 is a low-pin-count chip package 100 according to a preferred embodiment disclosed in R. O. C. Publication No. 348306 entitled "Device Having Resin Package And Method Of Producing The Same". The low-pin-count chip package 100 includes a chip 110 sealed in a package body 120. The active surface of the chip 110 is provided with a plurality of bonding pads 110a electrically connected to a plurality of connection pads 130. The backside surface of the chip 110 is exposed from the package body 120 through a conductive adhesive layer 112. The connection pads 130 are located at the periphery of the chip 110 and exposed from the lower surface of the package body 120 for making external electrical connection.
R. O. C. Publication No. 348306 entitled "Device Having Resin Package And Method Of Producing The Same" also discloses a method for making the low-pin-count chip package 100. The method mainly utilizes a metal frame 170 (see FIG. 2) to fabricate a plurality of the low-pin-count chip packages 100 simultaneously. The method comprises the steps of: (A) applying a photoresist layer over one surface of the metal frame 170, pattern transferring, and developing in a manner that areas on the metal frame 170 at which it is desired to form the connection pads 130 are not covered by the photoresist layer, (B) plating a layer of metal such as gold or palladium on the areas on the metal frame 170 without protection by the photoresist layer; (C) stripping the remaining photoresist; (D) attaching the backside surface of the semiconductor chip 110 onto the metal frame 170 through an adhesive layer wherein the active surface of the semiconductor chip is provided with a plurality of bonding pads 110a; (E) electrically coupling the bonding pads 110a on the semiconductor chip 110 to the corresponding connection pads 130; (F) forming a package body over the semiconductor chip 110. Finally, a separation step is performed to remove the metal frame 170. As shown in FIG. 2, the separation step typically comprises selectively etching the metal frame 170 with the connection pads 130 remaining intact by an etching agent.
Since the package body 120 does not cover the exposed lower surface of the connection pads 130, it can not firmly lock the connection pads 130. Adhesion depends on the overall nature of the interface region. A method for promoting adhesion is increasing the area of the interface between the package body 120 and the connection pads 130. However, since the connection pads 130 are formed by plating, the thickness thereof is limited (is proportional to the time for plating). Typically, thickness of the metal plating is only about 0.4 to 0.8 mil, which contributes quite little to the adhesion between the package body 120 and the connection pads 130.
Moreover, the connection pads 130 are made of metal with good electrical conductivity such as copper but the package body 120 is made of insulating material such as epoxy molding compound. Accordingly, the bond between connection pads 130 and the package body 120 is relatively weak and the difference of the coefficient of thermal expansion (CTE) therebetween is very large. Because of the CTE mismatch, stresses are induced at the interface between the connection pads and the plastic package body as the conventional package experiences temperature cycling. The stresses, in turn, result in the delamination at the metal-plastic interface. When the delaminations had occurred at the plastic-metal interface, moistures from the environment are easy to penetrate into the plastic package body and accumulate in the delaminated area. Once moisture accumulates in the package, rapid temperature ramp-up will cause the moisture to vaporize and expand, thereby inducing an hygrothermal stresses in the delaminated area which causes the surrounding plastic package body to popcorn. One of the most common occurrence of package popcorning occurs when the package described above is surface-mounted to a printed wiring board during the Infra-Red reflowing process.
Therefore, there is a need for methods of making the low-pin-count chip package that allow a significant increase of the thickness of connection pads thereby increasing the area of the interface between the package body and the connection pads. Thus, by promoting the adhesion between the package body and the connection pads, the present invention provides such a method overcoming or at least reducing the problems as described above.